1. Field of the Invention
This invention relates to a combination of mild hydrotreating and rapid thermal cracking of a vacuum resid. It especially relates to fluid bed coking or rapid pyrolysis for carrying out the thermal cracking.
2. Review of the Prior Art
Residual petroleum oil fractions, such as those heavy fractions produced by atmospheric and vacuum crude distillation columns, are typically characterized as being undesirable as feedstocks for most refining processes, due primarily to their high metals and sulfur content.
Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes being present. Additionally, trace amounts of zinc and sodium are found in most feedstocks. As the great majority of these metals, when present in crude oil, are associated with very large hydrocarbon molecules, the heavier fractions produced by crude distillation contain substantially all of the metals present in the crude, such metals being particularly concentrated in the asphaltene residual fraction and associated with large organo-metallic complexes such as metalloporphyrins and similar tetrapyrroles.
The residual fraction of single stage atmospheric distillation and two-stage atmospheric/vacuum distillation also contains the bulk of the crude components which deposit as carbonaceous or coke-like material on cracking catalysts without substantial conversion. These are frequently referred to as "Conradson Carbon" from the analytical technique of determining their concentration in petroleum fractions.
Coking is one of the refiner's major processes for converting residuals to lighter, more valuable stocks. Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high-boiling hydrocarbons, particularly residues obtained from cracking or distillation of asphaltenic crude distillates. The hydrocarbons generally employed as feedstocks in the coking operation usually have an initial boiling point of about 380.degree. C. (700.degree. F.) or higher, an API gravity of about 0.degree. to 20.degree., and a Conradson Carbon residue content (CCR) of about 5 to 40 weight percent.
The coking process is particularly advantageous when applied to refractory, aromatic feedstocks such as slurry decanted oils from catalytic coking and tars from thermal cracking. In coking, the heavy aromatics in the resid are condensed to form coke. During coking, about 15-50 wt.% of the charge goes to form coke. The remaining material is cracked to naphtha and gas oil which can be charged to reforming and catalytic cracking.
Hydrotreating resids before coking has long been practiced in order to desulfurize and/or demetalize the resid and produce higher-grade products, especially purer coke that is useful for making electrode carbons. For example, U.S. Pat. No. 2,963,416 describes a process in which 1-20 moles of hydrogen per mole of feed are added at 200-1000 psi and 300.degree.-1200.degree. F., suitably using a cobalt-molybdenum catalyst on alumina. The metals are deposited on the catalyst. After fractionating, coking is done at 750.degree.-900.degree. F. and 15-200 psig. Coke makes of 28% and 39% are described.
U.S. Pat. No. 2,871,182 discloses a process for mildly hydrogenating a resid and coking the hydrogenated material. A mixture of resid and 1.5-25 mols hydrogen per mol resid (approximately 300-5000 scf H.sub.2 /bbl feed) is reacted at 600.degree.-800.degree. F., 100-3000 psi, and 0.1-10 space velocity and is then coked at 800.degree.-1200.degree. F., 0-3000 psi, and with 0-5000 scf/bbl H.sub.2 to produce a fine, granular coke which is in slurry form and at least partially desulfurized.
U.S. Pat. No. 3,617,481 relates to a combination of coking and hydroprocessing of a resid having a high Conradson Carbon content and a high metals content in which the coke produced serves as catalyst base for the hydroprocessing step. Hydrotreating occurs at 725.degree.-950.degree. F. and 800-3000 psi if cracking is desired or at 550.degree.-800.degree. F. and 600-1500 psi if only desulfurizing is desired, using 1000-5000 scf/bbl H.sub.2. The coke make is typically 45-55% by weight, using fluidized coking.
U.S. Pat. No. 3,891,538 relates to hydrodesulfurizing an atmospheric resid, fractionating the product, and coking a mixture of the +1000.degree. F. product and decant oil produced by catalytically cracking the 650.degree.-1000.degree. F. fraction and then fractionating the cracked product to obtain the decant oil as the bottoms. Increased yields of gasoline and jet fuel are obtained.
Other processes for hydrotreating followed by delayed coking are given in U.S. Pat. Nos. 3,773,653, 3,902,991 and 4,235,703. U.S. Pat. No. 3,773,653 is particularly interesting in that it uses a three-phase ebullient bed reaction zone for hydrotreating a resid at 1500-3000 psi, 750.degree.-840.degree. F., an LHSV of 0.3-1.5 volumes of feed/hr/volume of reactor with a suitable hydrotreating catalyst. It was found that between 30% and 60% conversion occured with sulfur and vanadium at minimum levels in the coke. Coking produced 30% coke from virgin vacuum resid and 30% from a 475.degree. F. fraction therefrom, equalling 13.5% of the original feed.
U.S. Pat. No. 4,235,703 relates to delayed coking of vacuum resids after catalytically demetalizing and then catalytically desulfurizing. Catalytic demetalation occurs over a vanadium-promoted alumina catalyst at at least 725.degree. F., a pressure of at least 1067 psi, an LHSV of no more than 0.25, and a hydrogen rate of 500-1000 standard cubic feet/bbl of feed.
U.S. Pat. No. 4,062,757 describes a thermal cracking process for resids by upflow with hydrogen through a packed bed of inert, non-catalytic, non-porous solids, preferably at 790.degree.-950.degree. F., 100-2500 psi, a hydrogen flow rate of 500-2500 scf/bbl and an oil residence time of 0.3-3 hours. If hydrodesulfurization is desired, a catalyst comprising at least one Group VI metal and at least one Group VIII metal on a non-cracking support, such as alumina, is used.
In general, if a resid is thoroughly demetalized and desulfurized, reaction conditions must be quite severe and hydrogen consumption must be large. Demetalization particularly requires a high temperature unless a specific catalyst is utilized, such as a large-pore catalyst for demetalation followed by a small-pore catalyst for desulfurization, as disclosed in U.S. Pat. No. 4,054,508.
Thereafter, when the hydrotreating has been followed by coking, there has generally been some improvement in yield of liquid products and, of course, a greatly improved quality of coke. However, the cost of operating at relatively high temperatures and pressures and especially the cost of hydrogen consumption has largely minimized the usage of such processes.
It would normally be expected that coking a hydrotreated resid would produce greatly improved liquid yields at the expense of the coke make. However, this desirable result occurs only to a limited extent and, although there is increased saturation of products, much of the newly acquired hydrogen appears to be readily split from the hydrotreated resid when it is exposed to the high temperatures of coking. Moreover, the thermally cracked liquid products from the coker have a higher molecular weight than is desirable, thus minimizing the coker naphtha yield in favor of the heavier coker gas oils. Another undesirable result of many of the prior art processes is that the metal contaminants tend to be deposited on the catalyst, thereby forcing expensive replacement and/or regeneration thereof.
There is, accordingly, a need for a process in which a minimum of hydrogen is used to produce a maximum of lower-boiling liquid products and a minimum of coke containing as much of the contaminants as possible in order to isolate these contaminants from the catalyst and the hydrocarbon products that are desired.